This invention relates to an outboard motor, and more particularly to a silencing exhaust relief system for an outboard motor.
In an outboard motor employing a vertical drive shaft two-stroke cycle engine, it is common to discharge exhaust into a chamber formed in the drive shaft housing for providing exhaust tuning to the engine. An exhaust discharge passageway is in communication with the exhaust chamber for discharging exhaust therefrom to a submerged exhaust outlet, typically through the propeller hub. This arrangement has been found to provide highly satisfactory operation under most operating conditions in which sufficient exhaust pressure exists to ensure discharge of exhaust through the submerged outlet. However, under some operating conditions, such as at idle or in reverse operation, there is insufficient exhaust pressure within the exhaust system to expel water therefrom. Accordingly, water collects within the exhaust discharge passage downstream of the exhaust chamber, as well as in the chamber itself. In such situations, the outlet of exhaust into the exhaust chamber is typically submerged.
Various systems have been devised for providing relief of back pressure caused in the abovedescribed situation. Two such systems are disclosed in Harralson et al U.S. Pat. No. 3,967,446 and Freund et al U.S. Pat. No. 4,668,199, the disclosures of which are hereby incorporated by reference. The present invention provides yet another satisfactory system for relieving exhaust back pressure during idle, low speed or reverse operation, and also provides noise attenuation under such circumstances.
In accordance with the invention, an exhaust relief system is provided for an outboard motor. The outboard motor includes a power head having an exhaust discharge opening and a drive shaft housing mounted below and supporting the power head. An exhaust chamber is provided within the drive shaft housing, and an exhaust flow path is disposed downstream of and in communication with the exhaust chamber for providing a submerged exhaust outlet. A primary exhaust discharge passage is in communication with the power head exhaust discharge opening, and has an outlet for discharging exhaust into the exhaust chamber. The exhaust relief system is operable when the exhaust flow path is filled with water under certain operating conditions to prevent exhaust flow therethrough. The exhaust relief system comprises a first passage having an inlet at the upper end of the exhaust chamber and in communication with the exhaust chamber for receiving exhaust therefrom. An expansion chamber is associated with the drive shaft housing, and has an inlet for receiving exhaust from the first passage. A second passage is provided at the upper end of the exhaust chamber, and has an inlet for receiving exhaust from the expansion chamber. An outlet is in communication with the second passage for discharging exhaust to atmosphere. In a preferred embodiment, a drive shaft housing plate is adapted for placement at the upper end of the drive shaft housing. The first and second passages are preferably formed in the drive shaft housing plate, as are the inlets thereto. The drive shaft housing plate forms the upper wall of the exhaust chamber, as well as of the expansion chamber. An exhaust extension plate is adapted for placement between the drive shaft housing plate and the power head. The lower face of the exhaust extension plate preferably forms the upper wall of the first and second passages. The expansion chamber is preferably an annular space formed about substantially the entire periphery of the exhaust chamber. The inlet to the expansion chamber and the inlet to the second passage are arranged so as to be disposed substantially opposite each other. With this arrangement, it is assured that exhaust within the expansion chamber travels a maximum distance prior to entry into the second passage inlet, in order to attenuate noise as much as possible.
The invention thus provides a tortuous path having a series of expansion and contraction areas for dissipating energy in the form of noise from the exhaust as is it is discharged through the relief system. Accordingly, quiet operation at idle is achieved.